Packaging materials with a foam layer of e.g. polypropylene in the form of quasi endless rolled webs are used for producing thermoformed self-supporting packaging items as e.g. trays for food. Therefore these kinds of packaging materials are formed on so called “form-fill-seal” packaging machines (“FFS-machines”) to packaging items. According to this method the web of packaging material is fed into the machine in which it is formed into a web of trays which trays in the same machine are filled and sealed with a preferable transparent sealing film. Then usually the trays are separated from each other.
In the known packaging materials the polypropylene foam layer is carrying on at least one of its surfaces further layers. Depending on the composition and the thickness of these layers these layers may also serve as further means for increasing the stiffness as well as protection of the foam layer surface or as gas and/or aroma barrier and/or as surface layer for bounding the packaging tray and a preferable transparent lidding film.
Packaging materials with a polypropylene foam layer for producing trays by thermoforming are disclosed e.g. in EP-A-0 570 222. These materials comprise a foam layer and a multilayer film which multilayer film may include a barrier layer rendering the packaging material gas- and/or aroma tight. The properties of this multilayer packaging material need some improvement as far as the use of this material as self-supporting packaging items is concerned.
Therefore it is has been proposed to bond the foam sheet with the multilayer film by means of a bonding layer of a polyolefin based on the main monomer of the polyolefin of the foam layer (WO 96/25290). This results in an excellent adhesion between the laminate and the foam layer. However the packaging items especially self-supporting ones produced from this known multilayer film composition are not rigid enough for some applications.
Therefore there is a need to improve the stiffness of such packaging items, preferably trays, without increasing the thickness of the packaging material. Besides improving the mechanical properties of the packaging material there is also a demand to provide a packaging material suitable for thermoforming over a broader temperature range in order to achieve a faster production of packaging items.